WordPress is from 2003 and has been very successful since the beginning. FaceBook is from 2004. Both were PHP apps because the late 90s and early 2000s were the years of PHP CMSes and ecommerce platforms. Even if FaceBook did not happen PHP would have been one of the top 5 languages of that age. PHP was popular because of web hostings and the simplicity of apache + mod PHP. It was not big in hype because it was a really bad language until about version 7 and few people would admit to like it.

Actually, FaceBook worked against WordPress and the adoption of PHP because a number of people that could have used a WP instance to blog or to market a product started using a FB page instead. Ecommerce went from self hosted (Magento, Woocommerce, Prestashop) to hosted or to Amazon and also FB.